A01: Pulsed detonation combustion in a Multi-Tube-Configuration

Summary

Pulse detonation combustion will be investigated experimentally on a multi-tube test stand and the influence of various ignition timing strategies on the interaction between the detonation tubes and a plenum on the turbine side will be characterized. The interaction between tubes and with various plenum geometries will be examined at continuous operation conditions. The influence of increased combustor inlet temperatures and pressures on the deflagration to detonation transition will be studied using the single-tube configuration from phase 1.

2nd Funding period 2012 - 2016

Preliminary work

The work packages of the subproject A01 include enhancing the setup of the single-tube configuration, developed during the first funding period, as well as the construction of a multi-tube setup. Reliable operation of the detonation chamber one of the achievements of the first funding period. In order to generate detonations, two different geometries were used: (1) the installation of obstacles, such as orifice plates, significantly increases turbulence and flame stretching. Periodic repetition of this procedure enhances the spontaneous deflagration-to-detonation transition (DDT). Acceleration of the deflagration front through obstacles is widely used within detonation research to minimize the DDT run-up distance. However, the discontinuous change of the cross-section causes a substantial pressure loss, which evidently is a major throwback for the development of an efficient detonation combustor. (2) A lower pressure loss can be achieved through a convergent-divergent nozzle. The shock generated by the deflagration front is focused at this orifice, creating a very high local pressure, resulting in the formation of a detonation front.

Cell structure on the wall of the detonation tube, visualized by a soot-coated stainless steel foil

For the implementation of a pulsed detonation combustion, not only a reliable DDT, but also the highest possible firing frequency and a long operating time is of central importance. Each cycle consists of filling the detonation tube with a combustible mixture, combusting the mixture, and a purging process to separate the hot exhaust from the fresh combustible mixture of the next cycle. By adjusting the injection geometry during the second funding period, the purging process could be optimized, allowing operation with a firing frequency of up to 30 Hz. In addition, the installation of a water circuit to cool the detonation tube allows operation of the test bench for several minutes. During this time, a thermal equilibrium is achieved, which can be verified by constant temperature inside the tube. For the design of a detonation combustion chamber for gas turbine application, knowledge about dependence of the process on initial pressure and temperature is essential. The influence of the initial temperature on the properties of the detonation front was investigated for temperatures from ambient temperature up to 500 °C. The results were presented at the AFCC 2018 (Active Flow and Combustion Control). The size of the cells that form due to instabilities in the detonation front is a characteristic parameter of a detonation and provides information about the required diameter of the tube to allow a DDT. It could be shown experimentally that the detonation cell size decreases linearly with increasing temperature. In addition, the initial flame acceleration, which is linked to the DDT length, was investigated. Here it could be shown that the acceleration of the flame is reduced with increasing temperature, until a temperature of approx. 250 ° C, from where on it remains constant. By adapting the test rig, operation is possible at an elevated initial pressure of up to 3 bar. In future experiments, the influence of the initial pressure on the flame acceleration and DDT will be investigated.

Multi-tube setup

In a multi-tube setup, which is currently built up, the interaction between several detonation tubes will be investigated. In cooperation with subprojects A05 and C01, this test bench will be connected to a plenum. The occurring interactions between the detonation tubes should be quantified and used for the control of a stable and efficient operation.

1st Funding period 2012 - 2016

Pulsed detonation combustor

Summary

The focus of subproject A01 is the development, construction, and optimization of one of the primary experiments in the Collaborative Research Center, the pulse detonation combustor. In order to achieve reliable multi-cycle operation at high ignition frequencies, cooperation with the numerical, theoretical, and controls subprojects are required for shortening the length required for deflagration to detonation transition and determining a suitable fuel and air injection strategy. The data obtain during these experiments will be used as boundary conditions for other subprojects in the CRC.

In the preliminary work, high-speed visualization of the flame was conducted using laser sheet tomography. The purpose of these experiments was to determine the initial flame propagation speed and flame acceleration for various flow obstacles. In the process, it was determined that the shape of the obstacles is of less importance than the blockage ratio. The flame propagation for two different obstacles (a plate and an orifice) is presented in Figure 1. More experimental results are shown in Figure 2. Further results for an array of orifices are shown in Figure 2. The geometry for a new test stand was developed based on these experiments. This test stand is now installed in the new Energy Laboratory in the Department of Experimental Fluid Dynamics, which was financed by TU Berlin and opened in 2014. Alongside the laboratory for the subprojects A01 and A03 of CRC1029, several other laboratories were made available by the contribution of the university. One laboratory is being used for the investigation of thermoacoustic instabilities in gas turbines and aircraft engines. In another, a complete gas turbine was installed for the research of the thermodynamic cycles for innovative combustion processes, for instance, the injection of high amounts of steam into the combustion air.

For the current detonation combustor, two main configurations are possible. The first is a modular construction of tube sections and orifices, with which the number of orifices as well as the separation distance may be quickly and easily varied. The pressure of the detonation wave may be measured using fast response, piezoelectric pressure transducers in order to determine the detonation speed and pressure. This configuration is shown in Figure 3.

The alternative configuration consists of a rectangular canal, allowing for optical access through two acrylic glass windows. Optical measurement techniques, such as high-speed Schlieren and shadowgraph may be used. Various geometries may be installed on the upper and lower walls, in order to observe the effects of obstacles and dynamic injection on the flame acceleration. Examples of the high-speed shadowgraphs for laminar and turbulent injection conditions are shown in Figure 4.